They had the best technology NASA had available in
the 1970s when they were launched. The spacecraft
we are launching more recently, such as New
Horizons (the one that just went by Pluto),
are considerably more advanced. Voyager's power
source consists of a block of plutonium that is
more radioactive even than normal plutonium,
and which generates heat through radioactive
decay. This heat is used to power the
spacecraft. As it decays, the probes have less
and less power, and will eventually no longer be
able to send messages.
I don't know how the antenna that is used to
send radio signals sends a tight beam to Earth,
but certainly we can still see it.
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